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Spotlight on Optics

March 2012

Spotlight Summary by Andrey Kuzmin

The development of new laser sources that operate in the mid- and longwave-infrared wavelength range between 3.5 and 25 µm, also known as the molecular fingerprint region, is a task of great importance due to wide applications in atmospheric, biological and medical sciences as well as for security screening. Typical applications for such sources are IR spectroscopy, in-field spectroscopy of trace gases, military countermeasures for airplanes against heat-seeking missiles, etc. The sources used most frequently in these applications are semiconductor lasers – laser diodes (LDs) and quantum cascade lasers (QCLs) – given that they are compact and robust. The operating principle of LDs is based on the electrical injection of electrons and holes into an active region of semiconductor material, where they recombine yielding photons of wavelength close to the band-gap of the active region. In a QCL, instead of using opposite-charge carriers in the conduction and valence bands, only electrons participate in the process of creating coherent photons by making transitions between upper and lower energy subbands in the conduction band. The subband energy levels are designed by using quantum confinement in nano-layered structures to create the active region. To achieve the cascading emission of photons, an appropriate bias voltage between the device’s electrodes is applied, and electrons are injected into the active regions of multiple stages interspersed with the doped electron injectors.

In the past decade, the main effort has been directed towards reducing the injection current threshold for laser operation. High thresholds imply large input electrical powers and result in significant heating of the active region, which is detrimental to the laser operation. While QCL threshold power densities have remained high, there has been dramatic progress in the development of mid-infrared emitters with much lower thresholds using the so-called interband cascade laser (ICL), which combines the interband nature of the active transition with the multi-stage cascading inherent in the QCL design. The ICL current threshold is typically limited by non-radiative Auger recombination, which in the mid-infrared turns out to be far weaker than the intersubband scattering that dominates the QCL threshold. The authors of this Optics Express paper have demonstrated that their recent reductions in the ICL threshold, achieved by rebalancing the electron/hole density ratio in the active region by heavily n-doping the electron injectors, can be extended to longer emission wavelengths of lambda = 4.7 and 5.6 µm. While previous ICLs in this spectral range were limited to cryogenic operation, the improved devices operate in continuous-wave mode at room temperature and lase up to 60 C. The power-density thresholds are approximately an order of magnitude lower than in state-of-the-art quantum cascade lasers in this spectral range. All these developments will contribute noticeably to the compactness, portability, and lifetime of laser-based sensing systems in the mid-infrared.